1. Field of the Invention
The present invention generally relates to improvements in identifying, sensing, and tracking of objects in predefined spaces by electromagnetic means.
2. Background and Prior Art
There exists a great and increasing need to identify and to automatically detect the presence, orientation and movement of objects in a predefined area. Such objects can include a wide range of animate and inanimate things, such as people""s hands, pill bottles, tools, packages, toys, and many more. The applications for such automatic object detection and identification are numerous, including detecting a person""s activity over a work surface or interactive display; monitoring the inventory or theft of products on a retail store shelf; locating and aligning parts for robotic assembly in manufacture; and recreational games or toys relying upon placement and movement of objects, to name a few.
One common method of identifying and tracking objects employs a visible xe2x80x9ctagxe2x80x9d or target marking that is affixed to the desired object and uses a computer vision tracking system to monitor the object. U.S. Pat. Nos. 6,301,763, 5,828,770, and 4,672,562, incorporated herein by reference, disclose various machine vision methods for identifying and tracking the 3D movement of objects. While the low cost of such optical tags and markers is an attractive feature, the cost of the camera and vision system is not sufficiently low for many consumer applications. Additionally, vision-based systems require line-of-sight access between the visible marker and the detector, which is difficult to employ in non-controlled environments and lighting conditions, such as for home or retail applications.
Another method of detecting and tracking objects employs an xe2x80x9cactivexe2x80x9d electronic tag affixed to the object, which contains circuitry for transmitting a radio frequency signal that can be used by the host unit to determine the object""s identity, presence, orientation or movement. As an example, U.S. Pat. No. 6,204,764, incorporated herein by reference, discloses a method for identifying and tracking objects such as key cards and folders inside a filing cabinet drawers. Although radio-frequency tagging methods are robust and do not require line-of-sight, the expense of the electronic circuitry inside these tag makes this approach prohibitive for low-cost applications.
As a means of reducing cost, it is also possible to tag an object with an electromagnetically resonant structure, such as an xe2x80x9cLCxe2x80x9d resonator having an identifiable and detectable resonant frequency and Q factor. U.S. Pat. Nos. 6,020,849 and 5,386,219, incorporated herein by reference, provide a method for tracking such tagged objects over a surface using an electronic sensing array of coils or electrodes. This technology is commonly used in digitizing pen tablets and computer automated design (CAD) stylus input devices. While the electromagnetic tags employed in such systems are relatively low cost, the electronic switching circuitry and wiring required to multiplex the signals throughout the sensing array make these systems too costly for simple mass market products such as retail store displays, a home medicine cabinet shelf, or children""s toys.
Other methods for identifying objects have also been invented in the field of anti-counterfeiting and authentication. U.S. Pat. Nos. 5,434,917 and 3,878,367, incorporated herein by reference, disclose methods for identifying and authenticating credit cards through the use of randomly dispersed embedded magnetic particles that are detected as the credit card is swiped past a specialized scanner. U.S. Pat. No. 3,519,802, incorporated herein by reference, describes a method for electromagnetically marking a credit card through the use of embedded conducting plates; detection circuitry for this invention is not described. U.S. Pat. No. 5,992,601, incorporated herein by reference, discloses an apparatus for identifying and sorting currency that has been tagged with specific patterns of magnetic ink. While such anti-counterfeiting and authentication technologies provide a rudimentary means of identifying objects, the detection means is generally limited to specialized scanning and sorting apparatus with fixed geometries, and not adaptable or scalable to surfaces such as table tops, desks, shelves, retail display fixtures, or game boards. Additionally, these inventions do not provide a means for tracking the position or orientation of objects over a surface.
The present invention provides a method of identifying objects over a surface and a means for determining the position and orientation of the specified objects with respect to the sensing surface. The present invention combines the low-cost advantage of using vision-based tags (e.g. barcodes) with the versatility and security of electronic tagging methods. Furthermore, the present invention can be easily implemented with a conventional manufacturing process, such as printing or attaching a label, and still achieve the ability to operate via non-optical means, such as through a table-top or through packaging material such as an envelope, plastic casing, or product label.
The present invention entails the use of a reference surface, comprised of an array of electrodes or coils generating electromagnetic radiation having a characteristic frequency of oscillation, typically in the range 1-50 MHz. The objects in proximity to the sensing surface couple electromagnetically to the array of electrodes and coils, which then alters the characteristic frequency of one or more elements in the array. The resulting frequency shifts are thus an indirect measure of the electromagnetic response of the object. By monitoring the individual frequency shifts among the array elements, one or more objects in proximity to the surface can be sensed.
Changes in the characteristic frequencies of the elements are measured using a threshold detector and frequency counter, both of which can be easily implemented using a low-cost microcontroller, for example. Although changes in the sensing field induced by the object can also be monitored by other standard methods, such as measuring changes in the amplitude or phase of the sensing voltage, the technique of frequency-counting is much more inexpensive, requiring a minimal number of electronic components.
For most applications of this invention, the area of each sensing element (electrode or coil) in the sensing array would typically be in the range of 0.1 square centimeters to 100 square centimeters. A densely-spaced array of small sensing elements provides better lateral resolution; however, larger sensing elements provide a greater detection distance, since they produce probing fields that extend a further distance perpendicular to the array surface. In a given application, a combination of large and small sensing elements can also be used.
In the first embodiment of the present invention, rudimentary identification and detection of each object is accomplished by noting the intrinsic electromagnetic response of the material comprising the object (for example, distinguishing between a plastic box, a metal box, or container of milk). The characteristic electromagnetic properties of the objects can be intrinsic to the object or can be engineered, such as through the use of electrically conductive, magnetic, or dielectric materials applied to the object.
More significantly, for the case of a plurality of objects having similar electromagnetic properties, such as a stack of magnetic-ink-coated paper, this embodiment enables the ability to estimate the quantity of objects present. This ability is particularly relevant for inventory-monitoring applications, such as counting the number of greeting cards in each pocket of a display rack, or counting the number of magazines in a vending machine row.
In a preferred embodiment of the present invention, a quantifiable method of object identification is further achieved through the use of electromagnetic markers, such as electrically conductive or magnetic patches, that can be either printed or embedded onto (or into) the object in a specific coded pattern. Given a known electromagnetic response and spatial distribution of the markers, the object can be uniquely identified and its 2D orientation ascertained. Using a large array with greater surface area, multiple objects can also be detected and identified simultaneously.
The present invention also provides a means for determining the lateral position of a specified object or objects. Given an electrode array that is larger than the size of the objects and/or electromagnetic marker pattern, the position of the object can be determined within a resolution limited by the density and number of electrodes in the array. In general, the sensing elements (electrodes or coils) should be closely spaced in order to increase lateral positioning resolution; however, each sensing element must be sufficiently large to adequately couple electromagnetically to the object over the surface.
The noted method also provides a means to determine the orientation of the object relative to the sensing surface. A non-rotationally symmetric 2-dimensional pattern of markers embedded in the surface of the object facing the sensing surface enables the system to determine the 2D orientation of the object. By affixing or embedding the aforementioned electromagnetic markers to all faces of the object""s surface, such that each face is distinguishable from every other face, it then becomes possible to obtain a simple determination of the object""s 3D orientation. By way of example, one application of this technique could be a system or method for determining the orientation of a die in a casino board game.
A further embodiment of the present invention involves the use of a second layer of electrodes that are used to modulate or mask-off portions of the electromagnetic field produced by the sensing array. This xe2x80x9cmodulation layerxe2x80x9d thus enables the surface array of the array to be scanned electronically by activating or xe2x80x9cunmaskingxe2x80x9d only one section of the sensing array at a time. Although it is possible to use a conventional approach to multiplex the radio frequency electronics to the sensing array, the cost of a multiplexer and switches for these radio frequency signals is prohibitive for low-cost applications. Therefore, the a motivation for the masking layer of the present invention is to the lower cost and complexity of the system by providing a means for a single set of radio-frequency electronics (oscillator, frequency counter) to selectively monitor each of the potentially large number of electrodes in the sensing array. In addition, careful control of masking enables the system to have an additional degree of control with which to create a spatial mapping of the electromagnetic properties of the object. Note that in order to modulate or mask the sensing field, the electrodes in the modulation layer only need to be connected to electrical ground and do not need to carry a radio-frequency signal. Therefore, this embodiment makes it possible to scan the sensing array using simple low-cost electronic switching components.
Finally, in a further embodiment of the present invention, the modulation layer mentioned previously can also be employed to convey digital information to an electronic device that is proximal to the sensing array. It is noted that the operation of the modulation layer to mask or unmask portions of the array inherently produces a two-state amplitude modulation of the sensing field (corresponding to switch open or switch closed). Any electronic device tuned to the particular frequency of the array, such as 13.56 MHz, can readily detect and monitor these amplitude modulations. Thus, by modulating the sensing field in a recognizable time-sequence, the sensing array can convey digital information to the proximal electronic device.